Push switches used in input operation sections of electronic apparatuses are demanded to have small sizes and excellent durability, and are further desired to operate by an operating force predetermined to prevent malfunction due to accidental touch on the operation sections and to generate a proper click feel. Therefore, the push switches often includes elastic members made of rubber and having conical shapes.
A conventional push switch disclosed in Japanese Patent Laid-Open Publication No.10-92260 and Japanese Patent Laid-Open Publication No.11-265634 will be explained. FIG. 7 is a front sectional view of the conventional push switch, and FIG. 8 is a perspective exploded view of the switch. A contact substrate 1 includes a peripheral fixed contact 2, a central fixed contact 3, and connection terminals 2A and 3A connected with the contacts 2 and 3, respectively. A movable contact 4 of elastic metal thin plate is provided over the contact substrate 1 and includes a ring 4A and a tongue 4B. The ring 4A is mounted on the peripheral fixed contact 2. The tongue 4B projects toward the center of the ring from an inner periphery of the ring and is folded upward. The contact substrate 1 and the movable contact element 4 provides a switch contact section.
A rubber elastic element 5 is placed on the movable contact element 4. The elastic element 5 includes a central columnar portion 5A, a conical portion 5B, a ring portion 5C, and a protrusion 5D. The conical portion 5B having a predetermined thickness flares linearly downward obliquely from a junction 5E at the outer circumference of the columnar portion 5A. The ring portion 5C has a lower end mounted on the annular portion 4A of the movable contact element 4. The protrusion 5D having a diameter smaller than that of the columnar portion 5A faces the tongue 4B of the movable contact element 4 provided at the lower end of the columnar portion 5A. The conical portion 5B is hollow and has a truncated cone shape.
A push button 6 made of rigid resin as an operation section is located on the top of the columnar portion 5A of the elastic element 5. A case 7 is fixed by a pawl 1A of the contact substrate 1 so as to surround the periphery of the conical portion 5B. The button 6 is supported by a guide groove 7A in the case 7 so as to be movable up and down.
FIG. 9 is a front sectional view of the conventional push switch which is operating.
In this push switch, when the top of the push button 6 is pressed by a pressing force F, the columnar portion 5A and protrusion 5D of the elastic element 5 are pushed downward with the button 6. Then, as shown in FIG. 9, the conical portion 5B elastically deforms outward by a predetermined stroke with a click feel, and the leading end of the protrusion 5D pushes the tongue 4B of the movable contact 4 to have the contact 4 contact the central fixed contact 3. This contacting allows the peripheral fixed contact 2 to be connected with the central fixed contact 3, and makes the contacts 2 and 3 to output a signal transmitted to a circuit of the electronic apparatus through the connection terminals 2A and 3A.
Then, when the pressing force applied to the button 6, i.e., the columnar portion 5A of the elastic element 5 is released, the conical portion 5B restores its original truncated conical shape with its own elastic restoring force, so that the button 6 is pushed upward with the columnar portion 5A. Simultaneously to this, the tongue 4B of the movable contact element 4 restores upward to its original shape, and is departed from the central fixed contact 3.
In the conventional push switch, after tens of thousand times of operations at a temperature extremely lower or higher than a room temperature, cracks may be generated in the junction 5E linked to the conical portion 5B linearly flaring from the outer circumference of the columnar portion 5A of the elastic element 5.
If the rubber forming the elastic element 5 has a hardness reduced by having the composition of the rubber vary, a durability of the rubber against deterioration of the rubber is improved, as disclosed in Japanese Utility Model Laid-Open Publication No.6-56929.
If the rubber forming the elastic element 5 has a small hardness, however, the conical portion 5B starts deforming with a small force, and thus, an operating force of the push switch becomes smaller.